In recent years, for the purpose of reducing environmental loads, strict regulations against exhaust gases have been successively introduced in the automotive industry, and the development of post-processing apparatuses of exhaust gases has been carried out. The exhaust gases contain, in addition to carbon dioxide (CO2) as a global warming substance, harmful substances, such as particular matters (PM), hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. Among these substances, very strict regulation values have been imposed on PM and NOx. As the measure for reducing an amount of these substances discharged, gasoline automobiles are provided with a three-way catalyst, whereas diesel automobiles are provided with a diesel particulate filter (DPF). The exhaust gases are cleaned by passing through these members, and then discharged into atmospheric air.
In recent years, it has been recently reported that the active sites of the three-way catalyst are poisoned with phosphorus components contained in a lubricating oil composition for internal combustion engines, thereby causing deterioration in a catalyst performance thereof. In addition, it has been reported that an ash originated from metallic components is deposited on the DPF, thereby reducing the service life of the DPF. For this reason, at present, in the ILSAC Standard and the JASO Standard as standards for a lubricating oil composition for internal combustion engines, the upper limits of the phosphorus content and the ash have been established, and the development of lubricating oil compositions for internal combustion engines in which the phosphorus content and the blending amount of the ash are reduced within the range of prescribed values have been advanced.
As an example of a method of reducing the ash, there is exemplified a method of reducing the amount of a metallic detergent. But, in the conventional lubricating oil compositions, the metallic detergent was essential from the viewpoint of improving the acid neutralizing properties, and hence, if the addition amount of the metallic detergent is reduced for the purpose of reducing the ash, there is a concern that detergency of the lubricating oil composition is lowered. In addition, if the addition amount of the metallic detergent is reduced, the acid neutralizing properties are lowered, and hence, there is also a concern that oxidation deterioration of the lubricating oil composition is liable to progress. On the other hand, there has been proposed a so-called ash-free detergent-dispersant as a replacement of the metallic detergent (see Patent Document 1). In addition, there has been proposed addition of a specified heterocyclic compound capable of preventing oxidation deterioration of the lubricating oil composition from occurring (see Patent Document 2).